1. Field of the Invention
The present invention relates to a wave gear device equipped with a tapered flexible externally toothed gear that can reduce the bending stress generated at the root-rim surface of the flexible externally toothed gear and increase transferred torque.
2. Description of the Related Art
Since its invention (Patent Document 1) by C. W. Musser until now, the wave gear device has had a variety of inventions and designs of the present device made by many researchers including the present inventor. There is a variety of inventions relating only to the tooth profile. For example, the present inventor proposes having the basic tooth profile to be an involute tooth profile in Patent Document 2, and proposes a tooth profile design method for defining the addendum tooth profile of a rigid internally toothed gear and a flexible externally toothed gear for performing wide-area contact using a method for approximating the meshing of the teeth of both gears with a rack in Patent Documents 3, 4.
In general, a wave gear device has an annular rigid internally toothed gear, a flexible externally toothed gear coaxially arranged on the inside of the internally toothed gear, and a wave generator fitted to the inside of the externally toothed gear. The flexible externally toothed gear comprises a flexible cylindrical part, a disk-shaped diaphragm extending in the radial direction from the rear end of the cylindrical part, and external teeth formed on the external peripheral surface of the front end opening side of the cylindrical part. The front end opening side portion of the cylindrical part of the flexible externally toothed gear is made to elliptically flex by the wave generator, and the external teeth at the two end parts along a direction of the major axis of the elliptical curve mesh with the internal teeth of the rigid internally toothed gear.
The external teeth of the flexible externally toothed gear that is elliptically flexed has increased flexing amount from the diaphragm side to the front end opening along the tooth trace direction. Each portion of the teeth of the flexible externally toothed gear repeats flexing in the radial direction in association with the rotation of the wave generator. The present inventor proposes a method for setting the tooth profile with consideration given to such flexing motions (coning) of the flexible externally toothed gear by the wave generator in Patent Document 5.
In the wave gear device proposed in Patent Document 5, an arbitrary axially perpendicular cross-sectional position of the tooth trace direction of the flexible externally toothed gear is defined to be a primary cross section, and a tooth profile of the flexible externally toothed gear and the rigid internally toothed gear that is able to continually mesh at the primary cross section are formed. Assuming the flexing amount of the external teeth of the flexible externally toothed gear to be proportionate to the distance from the diaphragm from rear end of the diaphragm side to the front end opening, addendum modification is applied on the portions of the external teeth other than the primary cross section in accordance with the flexing amount, and interference of tooth profiles of both gears is avoided.
In Patent Document 6, the present inventor proposes the use of a conical tooth profile for the external teeth of the flexible externally toothed gear to avoid interference of tooth profiles of both gears caused by the flexing of the flexible externally toothed gear at the locations other than the axially perpendicular cross section (primary cross section) defined at a predetermined position of the tooth trace direction of the flexible externally toothed gear of the wave gear device.
[Patent Document 1] U.S. Pat. No. 2,906,143
[Patent Document 2] JP-B 45-41171
[Patent Document 3] JP-A 63-115943
[Patent Document 4] JP-A 64-79448
[Patent Document 5] WO 2010/070712
[Patent Document 6] WO 2005/124189